The invention relates generally to logging in boreholes. Specifically, the invention relates to noise logging in oil and gas wells.
The flow of fluids across a pressure differential generates noise as the mechanical energy is converted to acceleration or turbulence. Borehole noise logging, a measurement of acoustic energy, is useful in the qualitative evaluation of fluid flow. In uncased boreholes, the noise log serves as a detection means for natural flow.
For some open hole boreholes, it is important to know whether or not there is sufficient porosity and extent to make a commercial reservoir, and then whether the reservoir when present has sufficient permeability to make the production economical. When these wells are drilled using compressed air and remain empty during the logging operation, a noise log indicating natural flow indicates that some permeability is present.
Also there are many formations which are producers only in spotty areas and are not usually considered as the primary horizon because they are a high risk situation. However, when found, these reservoirs may add substantially to the production or life of a particular well, or may be of some interest in the future. A downhole noise logging system which can detect natural flow is advantageous.
In cased wells, noise logging is valuable for identifying production through perforations, distinguishing perforation production from channel flow behind casing, and determining the presence of sand entry into the well, among other applications.
Noise logging devices used in the identification of noise sources are presently known which utilize a single transducer on a mandrel for logging through the oil well. Single transducer noise logging is discussed in an article entitled "Specialized Applications of Noise Logging," by R. M. McKinley, Society Petroleum Engineers, Paper No. 6784. Typically, the single acoustic transducer detects noise generated, for example, by a producing perforation and translates the acoustic energy into an electrical signal for transmission to the surface for recording. Using a single transducer, the log is typically created from station-by-station measurements. That is, the logging tool is periodically stopped for a reading, for example, every two feet. The periodic stopping of the device in the casing has been necessary to avoid the detection of "road noise."
Road noise is that generated within the casing by the tool itself while moving through the casing. In a single transducer device if the tool is not stationary before a reading, a true fluid flow noise signal may not be isolated and is often contaminated with the road noise signal. An important feature of the invention described below is that it allows continuous logging measurements to be made while suppressing road noise.
U.S. Pat. No. 4,114,721 issued to Edwin E. Glenn discloses a borehole noise logging system using two detectors, the signals of which are transmitted uphole and applied to an amplifier, spectrum analyzer, cross-correlator and recorder. The cross-correlator is used to provide an indication of the time differential to each transducer as a way of locating flow through or behind the well casing.